A common process for producing pulp from wood is the Kraft process in which the wood is cooked with sodium sulfide and sodium hydroxide. Efficient pulping requires that the pulping chemicals be recovered from the spent pulping liquor or "black liquor". Recovery is conventionally achieved by concentrating the liquor and then burning it in liquid form in a recovery furnace. The resulting ash or smelt is recovered from the furnace and converted by conventional techniques for reuse in the pulping process.
A major capital investment in a pulping liquor recovery system is the recovery furnace. Each furnace has a maximum heat capacity which places an upper limit on the throughput rate of a given material. The throughput rate cannot ordinarily be incrementally increased, and when the capacity of the furnace is reached, it becomes a limiting factor in the recovery system, so that it is then necessary to replace the furnace with a larger one. Thus, a process which permits increased throughput capacity for an installed recovery furnace is highly desirable.
Pulping liquor is typically concentrated to between 60 and 70 weight percent solids, and in some of the more modern systems to about 75 weight percent solids, for burning in a recovery furnace. In all cases, the significant water content of the liquor reduces the amount of useable heat that is produced by the recovery furnace. The water vapor from the liquor is usually discharged with the flue gas, resulting in the substantial loss of heat, as sensible and latent, from the system.
A further limitation to a system which burns pulping liquor in liquid form is that the concentrated liquor is not inexpensively stored and must be burned in a recovery furnace when it is concentrated. The recovery furnace must have sufficient throughput capacity to accommodate the pulping liquor that is produced and concentrated at peak times, meaning that at other times, the furnace operates at less than capacity. If a system could be adapted to accommodate the average amount of a concentrated pulping liquor available rather than the peak amount, a smaller, less expensive recovery furnace could be used.
Further reducing the water content of the pulping liquor until a handleable solid remains would be advantageous in several respects. The substantially reduced water content would minimize all of the above-described disadvantages of vaporizing water in a recovery furnace. Furthermore, a solid, in certain forms, can be stored and supplied to a recovery furnace as required, whereby the operation of the recovery furnace is not tied to the operation of the liquor concentrating apparatus and the solid may be supplied to a recovery furnace at a generally constant rate that is independent of the rate of solid production.
At the present time, there is no method commercially available or in industrial use for drying pulping liquor efficiently to a solid. As the pulping liquor changes from a liquid to a solid, it becomes very viscous and would clog up any apparatus which would remove additional water from the liquor in the manner of an evaporator or a concentrator. Furthermore, the solid must be provided in a form that is burnable in a recovery furnace, and preferably in a form providing substantial exposed surface area for more efficient burning.
Accordingly, it is a primary object of the present invention to provide a method for drying pulping liquor to a burnable solid form, specifically in a particulate form. It is also an object of the invention to provide a method in which energy expended in the drying apparatus is efficiently conserved to be used for initial concentrating of the liquor.
In accordance with the present invention, pulping liquor is dried to form pulping liquor solid particulates in a method utilizing a fluidized bed dryer in which superheated steam serves as the fluidizing and drying gaseous medium. Although the invention is described primarily in terms of drying black liquor from a Kraft process, the process is applicable to drying liquor from other pulping processes, such as soda liquor, where dehydration produces a burnable, but steam-interactive solid.
U.S. Pat. No. 4,377,439 describes a process in which inorganic particulates from the recovery furnace are supplied to a coater dryer, where, in a fluidized bed, pulping liquor is coated and dried thereon. The coated particles are then returned to the recovery furnace. The fluidizing and drying medium, in this case, is flue gas. This process produces solid particulates with non-burnable cores and burnable outer coatings. It requires the continuous transfer of the inorganic mass between the recovery furnace and the fluidized bed dryer, which is inherently energy inefficient.
U.S. Pat. No. 4,295,281 describes drying brown coal particulates in a fluidized bed dryer using superheated steam as the fluidizing and drying medium.
The dry flue gas used in the above-mentioned U.S. Pat. No. 4,377,439 is substantially non-interactive with the black liquor and the black liquor solid particulates that are being dried with flue gas. Similarly, the superheated steam used in the above-mentioned U.S. Pat. No. 4,295,281 is substantially non-interactive with the coal particulates. These patents do not teach conditions which are required for drying pulping liquor to burnable, but steam-interactive particles. They do not teach, for example, the effect of alkali content on permissible operating temperature in a fluidized bed dryer. They do not teach velocities of fluidizing steam which must be maintained to fluidize particles that are tacky due to interaction with steam.
In contrast, steam is substantially interactive with pulping liquor and pulping liquor solid particulates, creating difficulties in maintaining a fluidized bed. Specifically, the steam can interact to make the dried pulping liquor particulates tacky so that they have a tendency to agglomerate. Also, while it is necessary to provide a sufficiently high temperature to boil water from the pulping liquor, higher temperatures initiate pyrolysis of the pulping liquor, which pyrolysis also tends to agglomerate particulates. Accordingly, the invention provides specific parameters for operation of a dryer wherein a fluidized bed of pulping liquor and pulping liquor solid particulates is maintained using superheated steam as the fluidizing and drying gaseous medium.